Very high downstream power levels and internet and TV drop-offs at late-night hours.

Just recently, my Internet as well as my TV has been starting to cut-out during the late-night, early morning hours.

A couple of days ago, I checked my cable modem signal levels and was surprised at what I saw.

This was when my service just recovered shortly after sun-up a couple of days ago.  During the day, the downstream power levels will drop to around 11ish dBmV.    At late-night, early morning, when the levels start approaching 18 dBmV I start noticing the internet starting to cut-out, as well as major tiling (artifacting) on my cox cable TV service as well.

Any ideas what's causing these kinds of swings in the downstream power levels?  I don't have any amps on my line that I'm aware of (unless there are amp(s) present beyond the point where the cable comes from the green service box on the ground [whatever that's called])